Silicon and germanium are two common semiconductor materials used in electron transport devices. As an advanced silicon material, the silicon-germanium alloy thin films consisting of silicon and germanium are more and more widely used. The silicon-germanium alloy thin film materials are mainly used for improving the electron mobility and hole mobility in semiconductor devices, such as Metal Oxide Semiconductor Field Effect Transistor (MOSFET), Modulation Doped Field Effect Transistor (MODFET), Doped Channel Field Effect Transistor (DCFET) and so on. In addition, the silicon-germanium heterostructure can also be used in quantum effect devices, for example Resonant Tunneling Diode (RTD), making it very promising to be the next generation of high-speed silicon device.
There is a great need for applying the silicon-germanium thin film materials in photoelectric micro electro mechanical systems (MEMS), but there are few reports about it. Through controlling the components, some properties of the silicon-germanium alloy thin film can be regulated, such as electrical properties, mechanical properties, optical properties and so on, which are very important for the photoelectric MEMS. In order to make the most advantage of the optical properties of the MEMS, preparing high quality silicon-germanium thin film and learning its mechanical properties become extremely necessary.
The conventional silicon-germanium alloy thin films are usually prepared by methods such as electrodeposition, chemical vapor deposition, magnetron sputtering deposition and ion beam deposition (IBD).
The reference of electrodeposition can be made to Chinese Patent No. CN101880901B titled “Preparation method for silicon-germanium alloy film material” (Authorized Announcement No. ZL201010301123.0).
The reference of chemical vapor deposition can be made to Chinese Patent Application Publication No. CN104393121A titled “Oxygen-doped amorphous silicon-germanium film, heterojunction crystalline silicon solar cell and method thereof” (Application No. 201410581435.X).
Both magnetron sputtering deposition and ion beam deposition have some limitations. It is mainly reflected in erosion track and target poisoning on the surface of the target during magnetron sputtering deposition. In ion beam deposition, the ion beam is sent obliquely, shortening the service life of the target; the escaped ion beam contaminates the chamber material and the silicon-germanium thin film through sputtering deposition; low deposition rate makes it hard to perform deposition on thick silicon-germanium materials.
The photoelectric MEMS requires excellent physical properties of the corresponding semiconductor thin film materials which are influenced by the composition of the materials profoundly. How to adjust the composition of the thin film precisely to obtain thin film with excellent physical properties is a key technique in semiconductor thin film deposition technique.
Reference 1 “Fabrication of Silicon/Germanium superlattice by ion beam sputtering, Vacuum, Vol. 66, December 2002, P457-462”, discloses a method to form semiconductor silicon-germanium thin film on the silicon substrate by ion beam deposition, wherein two layers of silicon and germanium thin film with a total thickness of 300 nm is deposited and the roughness of the thin film is 1.08 nm shown in atomic force microscope 3D image.
Reference 2 “Structural and electrical studies of ultrathin layers with Si0.7Ge0.3 nanocrystals confined in a SiGe/SiO2 superlattice, Journal of Applied Physics, Vol. 111, October 2012, P. 104323-1-104323-4” discloses a method for preparing silicon-germanium thin film by radio frequency magnetron sputtering deposition. The method can prepare relative thin silicon-germanium thin film by co-sputtering method, obtaining relative fine structure and good electrical properties. However, the method cannot control the composition of the thin film precisely, and utilization ratio of the target is low, which shorten the service life of the target and is not conducive to wide application.
Circular magnetic field is used in magnetron sputtering deposition, which forces the secondary electrons hurdle along the circular magnetic field. Correspondingly, the area controlled by the circular magnetic field is the area with the highest plasma density, which causes a circular groove on the target, i.e. erosion track. Once the groove penetrates the target, the whole target is destroyed, leading to low utilization of the target. In addition, due to the instability of the plasma used in the magnetic sputtering deposition, uneven etchings are formed on the surface of the target, causing target poisoning, which inevitably leads to the doping of films in the target poisoning area, decreasing the purity of the thin film.
In ion beam deposition process, the ion beam is sent obliquely, which leads to uneven etchings on the entire target, shortening the service life of the target. Some ion beams can escape and sputter on the vacuum chamber materials, leading to the generation of impurity ions which can contaminate the silicon-germanium thin film. The target area of the ion beams is quite small, resulting in low deposition rate, which makes it hard to perform deposition on relative thick silicon-germanium thin films.